[clang] Add 'override' to virtual function overrides generated by ClangAttrEmitter

ClangAttrEmitter.cpp generates ParsedAttr derived classes with virtual overrides in them (which end up in AttrParsedAttrImpl.inc); this patch ensures these generated functions are marked override, and not (redundantly) virtual.

I hesitate to say NFC since this does of course affect the behavior of the generator code, but the generated code behaves the same as it did before, so it's NFC in that sense.

Differential Revision: https://reviews.llvm.org/D83616